1. Technical Field of the Invention
The present invention relates to an engine starter (i.e. starter for engines), and in particular, to an engine starter having an intermediate gear, in which torque of an output shaft of a motor is transferred to an internal combustion engine through a ring gear thereof by having the ring gear engaged with the intermediate gear to crank up the engine.
2. Related Art
As related art, Japanese Patent Laid-Open No. 2002-180937 (U.S. Pat. No. 6,647,812) discloses an engine starter having an intermediate gear. As shown in FIG. 4, this engine starter includes a pinion gear 110 for transmitting the motor torque to a ring gear 160 of an engine through a clutch 100, an intermediate gear 120 which is constantly in engagement with the pinion gear 110, and a retainer 150 which is in engagement with a boss portion 130 provided to the pinion gear 110 and with a boss portion 140 provided to the intermediate gear 120. The clutch 100 allows the intermediate gear 120 to move in the axial direction (leftward in FIG. 4) integrally with the pinion gear 110 through the retainer 150 for engagement with the ring gear 160 of the engine, so that the torque transmitted to the pinion gear 110 is transmitted to the intermediate gear 120 and further to the ring gear 160 to start the engine.
However, in the engine starter mentioned above, the retainer 150 has been required to have a large thickness in the axial direction, which has resulted in making the length of the engine starter problematically large as a whole. The reasons are as provided below. In the engine starter mentioned above, an outermost diameter of a step portion 170 provided on a non-ring-gear side of the intermediate gear 120 (rightward in FIG. 4) is radially distanced (by an area indicated by S in FIG. 4) from an outermost diameter of the clutch 100. Therefore, when impacts m1 and m2 are imparted to the retainer 150 by the engagement of the intermediate gear 120 with the ring gear 160, the impacts m1 and m2 cause a bending moment that acts on an area indicated by X in FIG. 4 in the retainer 150. The area indicated by X corresponds to a surrounding area defined by the outermost diameter of a surface of the step portion 170 formed in the intermediate gear 120 and opposed to the retainer 150, and by the outermost diameter of a surface of the clutch 100, which surface is on the side of the retainer 150. In order to prevent the retainer 150 from being deformed by the bending moment (stress), it has been required that its mechanical strength be increased by making the thickness of the retainer 150 large in the axial direction. Thus, it has been a problem that the axial length becomes large in an engine starter having an intermediate gear.